1. Field of the invention
The present invention relates to turbomachines in which the stator carries at least one circumferential stage of blades which are pivotable in order to vary their setting.
2. Summary of the prior art
Compressors and turbines are known in which the stages of fixed blades connected to the stator are capable of being pivoted about a radial axis in order to modify the flow characteristics of the compressed gases and thereby improve the output of the machine under certain rotor operating conditions by preventing surging. If the stator is regarded as a first annular structure, a second annular structure is provided which is movable axially on the first while being held concentric therewith by suitable guiding means, the second annular structure being pivotally connected to the free ends of levers carried by the blades so that axial movement of the second annular structure rocks the levers to pivot the blades. The connection between the second annular structure and the levers may simply comprise radial handles carried by the levers at their free ends, and grooves or channels in the second annular structure which receive the handles. Such an arrangement is described in French Pat. No. 1 230 393.
All constructions known by the inventors utilize mechanical transmission means to control the axial movement between the two annular structures, such as pivoted levers, connecting rods, pinions or screw and nut systems, which have certain drawbacks. In the first place they are cumbersome and heavy. Furthermore, they are not always reliable because of the risk of seizure resulting from friction and wear of the assembled parts, especially in machines which operate at high temperatures, as lubrication is difficult and, also, substantial thermal expansion is prejudicial to a satisfactory degree of precision in the adjustment of the parts. Finally, it is difficult to apply an even force around the entire circumference of the second annular structure, which increases the risk of jamming of the movable annular structure.